1. Field of the Invention
The present invention relates to a procedure for the purification of crude tetrabromobisphenol sulfone (hereinafter referred to as "tetrabromobisphenol S") by using solvent extraction, rather than recrystallization. Tetrabromobisphenol S is tetrabrominated 4,4'-dihydroxydiphenyl sulfone.
2. Description of the Prior Art
It is known in the art to purify certain diphenol compounds by use of recrystallization techniques. For example, German Patent Publication Nos. 2,758,564 to 2,758 ,566 teach the use of water and an organic co-solvent to purify certain crude diphenol compounds by using recrystallization. Recrystallization techniques are also described in Japanese Patent Publication Nos. 54/119,425 and 62/178,534. The diphenol compounds need to be purified to remove, undesired isomers and other impurities that would impair the physical properties of polymers made from such compounds.
Tetrabromobisphenol S has been found to be a difficult compound to purify. It is usually available as a crude product containing 5-20% or more of tri-, di-, and monobromo sulfones as impurities. Crystallization of the tetrabromo sulfone is difficult since it is insoluble in many solvents and it has a tendency to remain in solution in those cases in which it can be solubilized. One solvent which has been found to work well in a recrystallization procedure producing a good recovery of a high purity product is para-dioxane. However, this solvent is toxic and is not preferred for use for that reason.